I. Field of the Invention
This invention relates to a downhole milling tool and to a cutter therefore cutting tubular members downhole.
II. Description of the Prior Art
Downhole milling tools are well known and may consist of a tubular body having an axial aperture therethrough for circulation of cooling fluid and a plurality of equi-circumferentially disposed slots in the outer surface of the body. In each of the slots is located a pivotally mounted cutting blade and a means is provided for moving each of the cutting blades radially outwardly from the body. Each of the cutting blades has an outer surface facing the direction of rotation of the tool which is dressed with a cutting material. The dressing may be formed by crushed tungsten carbide chips which are randomly dispersed on the blade surface. The problem associated with crushed tungsten carbide chips is that because they have an irregular shape and size they are difficult to secure to the blade surface and such inconsistency in shape also leads to unpredictable performance in respect of swarf cutting configuration and rate of penetration. This problem has been alleviated by using tungsten carbide elements having a circular or rectangular cross-section, the tungsten carbide elements being brazed, soldered or welded to the blade. It has been found, with advantage, that if tungsten carbide elements of regular shape are used then they may be located on the blade both radially and in an axial longitudinal direction of the blade in a regular formation such that each element provides a negative rake angle, that is, in use each blade is angled downwardly and rearwardly with respect to the vertical direction of the blade taken in the direction of rotation when in use.
The known cutting elements of regular shape have been known to be provided with a protruding ridge or chip breaker, that is a projection which limits the length of swarf cut by the cutting edge of the element. However such known elements have only one chip breaker and once that is worn off the performance of the tool deteriorates until the next element with a new chip breaker is exposed.
It will be understood that the provision of a chip breaker is extremely desirable to overcome the phenomenon known as "birdnesting", that is long spirals of swarf that are cut from a tubular member being machined forming into a conglomerate mass which restricts the flow of mud about a tool and reduces the rate of penetration of the tool. It is therefore an object of this invention to provide a milling tool and a cutter therefore in which the foregoing problems are substantially mitigated.